1. Field of the Invention
This invention relates to tracking control devices for video tape recorders (VTRs).
2. Description of the Prior Art
A so-called wobbling type tracking adjuster has been previously proposed for a helical scan type VTR. In this tracking adjuster, a playback head is mounted on a bimorph leaf which can be deflected in dependence on a voltage applied to electrodes of the bimorph. By application of a suitable voltage signal to the bimorph, the playback head is vibrated with a very small amplitude. Because the level of the reproduced signal derived from the playback head is reduced whenever the playback head deviates to either side from the center of the recorded track, the direction of the deviation can be discriminated from the change of level. The error-correcting voltage signal to be applied to the bimorph can thus be derived, and application of this signal to the bimorph enables the playback head to be controlled accurately to trace the recorded track.
This previously proposed tracking adjuster will now be described in more detail with reference to FIG. 1 of the accompanying drawings, which shows the tracking adjuster in block form.
A magnetic tape 1 has had a signal recorded thereon by the so-called helical scan system; that is, the recorded tracks are formed to be skewed relative to the longitudinal direction of the magnetic tape 1. A rotary playback magnetic head 2 scans the recorded, skew tracks to reproduce the signal therefrom.
A sine wave signal generator 3 generates a sine wave signal which forms a head wobbling or dither signal which is supplied to a bimorph 5 through an adder 4. The magnetic head 2 is mounted on the bimorph 5, so the magnetic head 2 is caused to reciprocate in the direction perpendicular to the length of the recorded tracks in response to this sine wave signal. Consequently, the level of the reproduced frequency modulated (FM) signal from the magnetic head 2 is changed in dependence on the sine wave signal. The reproduced FM signal is supplied to a synchronous detector circuit 8 through an envelope detector circuit 6 and a low-pass filter 7. The envelope of the reproduced FM signal supplied to the synchronous detector circuit 8 is a signal modulated by deviation of the tracking between the magnetic head 2 and the recorded track.
The sine wave signal is also supplied to the synchronous detector circuit 8 through a delay circuit 9. The delayed sine wave signal is added to the envelope signal at the synchronous detector circuit 8. As a result, the synchronous detector circuit 8 produces a signal changing in accordance with the deviation between the magnetic head 2 and the recorded tracks, and supplies it to the bimorph 5 through a low-pass filter 10 and the adder 4.
In this way, when the scan of the magnetic head 2 deviates from a recorded track, a signal changing in response to that deviation is applied to the bimorph 5 so that the magnetic head 2 correctly scans the recorded track.
Thus, in this previously proposed tracking adjuster, the use of the synchronous detector circuit 8 enables the relation between the amount of tracking error or deviation in the scan and the detected output to be made linear enough to correct for the tracking error or deviation. However, the synchronous detector circuit 8 is complicated, and indeed the whole arrangement of the tracking adjuster is rather complicated. In one particular example thereof, the synchronous detector circuit 8 and the low-pass filter 10 together require four integrated circuits and fifty-seven passive components.